Quantum memory storage to help quantum communications go the distance

Michał Dąbrowski, Radek Chrapkiewicz and Wojciech Wasilewski at FUW have created an atomic memory (glowing green) for use with quantum technologies (Photo: FUW, R. Chrapkiewicz)

The technologies made possible by breakthroughs in quantum physics have already provided the means of quantum cryptography, and are gradually paving the way toward powerful, practical, everyday quantum computers, and even quantum teleportation. Unfortunately, without corresponding atomic memories to appropriately store quantum-specific information, the myriad possibilities of these technologies are becoming increasingly difficult to advance. To help address this problem, scientists from the University of Warsaw (FUW) claim to have developed an atomic memory that has both exceptional memory properties and a construction elegant in its simplicity.

The FUW researchers from the Institute of Experimental Physics claim that the new, fully-functioning atomic memory has numerous potential applications, especially in telecommunications where the transmission of quantum information over long distances is not as straightforward as the transmission of simple electronic data encoded on laser light and traveling through optical fiber.

This is because quantum information can't simply be amplified every so often along its path of travel as information digitally encoded on a laser beam can be. Instead, it is essential that the quantum information itself remain absolutely preserved in its original form to maintain its inherent security, and boosting the signal risks disrupting the quantum state and immediately rendering the transmission useless and unusable.

In this vein, the new memory may prove useful in providing a means to bring into reality the DLCZ quantum transmission protocol (DLCZ being the initials of the physicists from the University of Innsbruck and Harvard University who proposed it; Duan, Lukin, Cirac, and Zoller), enabling quantum information to be sent across long distances.

As an essential requirement for this protocol to work, quantum information transmitted must be stored at various relay points along the channel of communication. Up until now, the physical capabilities to realize the DLCZ protocol have been unavailable, but this new atomic memory may help solve that problem.

"The greatest challenge in the construction of our quantum memory was the precise selection of system parameters that would allow it to save, store and read quantum information effectively," says Dr. Wojciech Wasilewski of FUW, "We have also found a novel way of reducing noise during detection."

The primary component of the quantum memory is a glass chamber about 25 mm (1 in) in diameter and around 100 mm (4 in) long. Coated on the inside with rubidium, the container was evacuated of air and filled with krypton gas and the cell magnetically shielded to protect the interior from stray magnetic fields. When the tube was heated to around 90° C (194° F), pairs of rubidium atoms expanded to fill the inside of it, whilst the pressurized krypton gas acted as a noise reducer by dampening their movement.

To record and recover quantum information, the researchers used three horizontally polarized external lasers on the chamber: one was used to pump (excite) the rubidium atoms, another was used to write by creating spin-wave excitations on those atoms, and the third was used to apply a read pulse. The resultant multimode light was then passed through a series of filters and detected by a sCMOS high-speed camera.

In other words, quantum information stored in the memory used photons from the laser beam to "imprint" quantum spin states on many of the excited rubidium atoms. As a result of this interaction, other photons were emitted simultaneously and the detection of these verified that the information had been saved. Information stored in the memory was then retrieved using another laser pulse.

"Until now, quantum memory required highly sophisticated laboratory equipment and complex techniques chilling the systems to extremely low temperatures approaching absolute zero," said Radek Chrapkiewicz, a doctoral student at FUW and researcher on the project. "The atomic memory device we have been able to create operates at far higher temperatures, in the region of tens of degrees Celsius, which are significantly easier to maintain."

According to the researchers, this quantum memory could also be used to store quantum information of varying spatial modes (different vibrational frequencies) within the one container, thereby increasing the storage capacity. As a result, this device may well be able to act as an inline memory for a number of fiber-optic communications simultaneously.

"The stability of the quantum information stored in our memory lasts from a few microseconds up to tens of microseconds. You’d be forgiven for asking how such short-lived memory could be useful at all, but bear in mind that it depends on the application," said Michał Dąbrowski, also a doctoral student and researcher at FUW. "In telecommunications, microsecond timescales are sufficient to conduct several attempts at transmitting a quantum signal to the next relay station."

The research work received funding from the Polish National Research Center and the team's study was published in the journal Optics Express.